Since the fluidized catalytic cracking (FCC) process was first introduced in the 1940s, the FCC process hydrocarbon feed has been used as the fluidizing media in the reactor and also to convey the regenerated catalyst from the bottom of the reactor riser up to the reactor cyclone or disengaging vessel. In the typical riser cracking FCC system that is used worldwide today, the hot regenerated catalyst is mixed at or near the base of the reactor riser with feed. The hot regenerated catalyst acts as the heat source for vaporizing the feed and also acts as a catalyst to convert the feed to lower molecular weight product. This vaporization and conversion of the feed results in vapors which are used to convey the catalyst up the riser to the spent catalyst-reactor vapor disengaging vessel where the reactor vapors and spent catalyst are separated from each other. After separation, the spent catalyst flows downwardly into a catalyst stripper, then into the regenerator where it is regenerated and then back to the base of the riser to complete the circuit. The reactor vapors, after separation from the majority of the spent catalyst, are fractionated into the desired products.
The hydraulics of the typical FCC process are very well known and are based upon the fact that a dense bed of fluidized catalyst will develop a higher pressure differential than a dilute phase of fluidized catalyst. As an example, the normal fluidized dense bed, as employed in regenerators, reactor beds or strippers that have upward velocities of between 0.5 and 3.5 feet per second (fps) superficial velocity will have densities of between 15 and 40 pounds per cubic foot (#/ft.sup.3) which equates to 0.10 to 0.28 pounds per square inch (psi) per foot of dense bed height. The catalyst circulating lines, such as regenerated catalyst and spent catalyst standpipes, which normally have little or no added fluidizing media or no significant upward gas superficial velocity will have densities between 25 and 50 #/ft.sup.3 which equates to 0.17 to 0.35 psi per foot of height. The reactor riser with an average velocity of 50 fps and catalyst to oil ratios of around 7 pounds of catalyst per pound of oil will have a density of 3.2 #/ft.sup.3 with a 2/1 slip factor. This 3.2 #/ft.sup.3 density will result in a pressure drop across the typical 120 foot long riser of about 2.7 psi or 0.022 psi per foot. Note that the typical riser pressure drop per foot is about 13 to 6% of the pressure drop in the spent and regenerated catalyst standpipes. This hydraulic difference allows the catalyst in the fluid catalytic cracking unit to circulate.
There are many configurations for the FCC process. Some have the reactor higher than the regenerator or vice versa. Some have the reactor and regenerator at the same elevation. Usually, if the reactor and regenerator are at the same elevation, the design operating pressures of both vessels are the same. For those units where the reactor and regenerator are at different elevations, the higher vessel operates at a lower pressure than the lower vessel. Usually this difference in pressure is no more than 5-10 psi. As is readily apparent in the design of an FCC unit (FCCU), as in any other unit, once the unit is built there are definite limits as to what can and cannot be done in the existing equipment.
As is apparent from the age of many of the FCC units still in operation, the refining industry has been very creative in increasing the capacity of existing systems and utilizing new catalyst and process technology with minimum changes to the existing reactor and regenerator vessels.
My U.S. Pat. No. 4,985,136, issued Jan. 15, 1991, and entitled "ULTRA-SHORT CONTACT TIME FLUIDIZED CATALYTIC CRACKING PROCESS" describes an FCC system that differs markedly from the typical riser cracking FCC process employed in the worldwide refining industry. U.S. Pat. No. 4,985,136 is incorporated herein by reference in its entirety. This ultra-short contact time fluidized catalytic cracking system is known in the industry as the "Milli-Second Catalytic Cracking" or "MSCC" process. The MSCC process described in my patent does not have a riser reactor and does not rely on feed vaporization and conversion to circulate the catalyst. Instead, the MSCC system employs a dispersion medium, or media, to disperse the regenerated catalyst downwardly into the feed, which is injected essentially horizontally into the MSCC contactor, or reactor. This dispersed downward flow of regenerated catalyst and dispersion media is critical to the proper hydraulic operation of the MSCC system. Locating the regenerator above the MSCC contactor requires that the regenerator vessel be located at over 100 feet above grade. This is not always acceptable for large capacity units which require large vessels, or in areas of high wind loadings or earthquake zones.
In addition to the mechanical considerations of MSCC technology, ultra-short contact time FCC process conditions differ markedly from those in the conventional FCC process. The heat balance is one area of considerable difference. Because of the short contact time reaction system employed, the catalyst to oil ratio can be 25:1 or greater. This compares to conventional FCC processes, where catalyst to oil ratios of 4:1 to 10:1 are common. This higher than normal catalyst to oil ratio results in approximately a threefold increase in the amount of combustion products resulting from the burning of the carbonaceous deposits on the catalyst, or "inerts," entrained into the reaction section. Such inerts may include CO, CO.sub.2, H.sub.2 S, SO.sub.x (sulfur oxides), H.sub.2 O, N.sub.2 and the like which are not desired in a fuel gas. This increases the loadings in the downstream fractionation system and gas concentration unit and lowers the quality of the fuel gas produced while increasing the amount of fuel gas. Also, the higher circulation rates required in ultra-short contact time FCC results in increase pressure drop across the reactor riser which will limit the catalyst circulation capabilities of existing equipment or require more lift media to reduce the reactor riser density. This increase in lift media will increase the loadings on the whole reactor system and downstream fractionation, which may result in reducing the throughput of existing systems or drastically increase the cost of new systems.
Therefore, a principal object of the present invention is to remove the regenerated catalyst from the normal FCC circulation route so that it can be treated to improve its quality and to give an independent control of pressure balance to allow for higher catalyst to oil ratios without overloading the downstream equipment with lift media.
Another object of the invention is to greatly reduce, by dilute phase stripping with the lift vapor, the amount of inerts entrained with the regenerated catalyst into the reaction system. This will reduce the quantity of fuel gas produced in the MSCC process by 40 to 150%. Since the catalyst to oil ratio in the MSCC process is about three times that of a conventional FCC, the amount of inerts in the fuel gas product typically will be increased by about three times. If the inerts are not removed in accordance with the present invention, the inert content of the fuel gas produced by the MSCC process may be as high as 70%, so that the quality of the fuel gas product is greatly reduced.
Still another object of the present invention is to reduce the catalyst carryover from the FCC reactor into the main fractionator by employing a cyclonic separation of the circulating catalyst to eliminate a majority of the catalyst fines (undesirably small catalyst particles) from the reaction zone.
Yet another object of the present invention is to condition the catalyst to obtain yield and product benefits in the practice of an FCC process. There has long been a desire on the part of refiners to have the ability to maximize gasoline or distillate, as desired. Up to now, this ability has not realized as it normally required a change in catalyst activity to accomplish this yield shift. Now in accordance with the present invention, by selecting the proper media mixture one can change the catalyst activity at will by laying down greater or lesser amounts of coke on the circulating regenerated catalyst in a lift pipe as hereinafter described. Thus, one can obtain almost instant catalyst activity control in the operation of an FCC process and easily switch from maximum gasoline to maximum distillate.
A further object of the present invention is to enable the production of olefins and petrochemical feedstocks from a second hydrocarbon feedstock used as the lift media or as a portion of the lift media.
Another object of the present invention is to reduce the increase in coke yield associated with using a lift media other than hydrocarbon feed to circulate the catalyst into the reactor system of an FCCU.
In one embodiment the need for regenerator catalyst coolers can be obviated by use of the process of the present invention.
Yet another object of the present invention is to hydrate the regenerated catalyst so as to increase the circulating catalyst activity.
Another object of the present invention is to separate the reactor regenerator hydraulics so that the regenerator does not have to be located above the reactor.